The present invention relates to a camera having an exposure control device so designed that the light measurement, the exposure condition determination and the photographing operation are carried out by means of a release means including an electromagnetic element to be controlled by means of a plural number of switches operating successively with the release operation.
In the case of a camera provided with the exposure condition determining device in accordance with the recent electric control system, it is necessary to provide a mechanism for closing the light measurement switch for supplying current to the light measuring circuit, the calculation circuit and so on before the release operation of the camera, for which there exist the following systems;
(1) A system in accordance with which the light measuring switch is closed or opened by manually operating the switch operation member provided at a proper position on the exterior of the camera.
(2) A system in accordance with which the light measuring switch is closed by drawing the winding up lever out into position with a preparation angle and wherein the switch is opened by bringing the lever into the withdrawn position.
(3) A system in accordance with which the light measuring switch is closed with the first step of the push down operation of the release button and the release switch is closed with the second step of the push down operation of the release button.
Thus, if the operation of the light measurement switch is forgotton in taking a picture in the case of the camera of the first system, the camera does not carry out the photographing operation so that there is a danger of losing a shutter opportunity, while, if it is forgotton to open the light measurement switch after having taken a picture the power source is consumed even when the camera is not used. This wasted power, which is inconvenient. In particular, in the case of the camera of the first system, it is necessary to close and open the switch each time a picture is taken. This is also very troublesome.
On the other hand, in the case of the camera of the second system, the winding up lever is operatively engaged with the closing and the opening of the light measurement switch so that there is little danger that the operation of the light measurement switch in taking a picture will be forgotten. There is a danger, however that the winding up lever could be brought back in the withdrawn position by mistake. In this condition, release operation is impossible and the shutter opportunity will be lost. Further, it can be thought that the winding up lever remains at the position with a preparation angle when the camera is not used and the electrical power source is consumed in vain. Further, if the camera of the second system is provided with a motor drive device, the winding up lever can not be operated so that it is necessary to provide a mechanism for changing over the circuit so as to operate the lever by means of the separately arranged light measurement switch. This is not worthwhile because the mechanism as well as the circuit of the camera becomes quite complicated.
In the case of a camera of the third system type, the light measurement switch in operative engagement with the release button is used and therefore the switch is necessarily closed and opened with the release operation in taking a picture. The waste of the power source likely to take place for the first and the second systems never takes place since the light measurement switch is closed for a short time upon releasing the shutter so that the waste of the power source is remarkably small. A camera so designed that current is supplied to each part of the camera, provided with the electromagnetic release device of the third system has already been brought into practice.
In the case of the camera of the third system type in accordance with which the electromagnetic release device is started and the proper exposure is obtained by means of the exposure condition determining device, which is so designed that the light measurement switch is closed by means of the first step of the pushing down operation of the release button so as to start the current supply to the light measuring circuit, the calculation circuit and so on and then the release switch is closed by means of the second step so as to start the electromagnetic release for actuating the photographing operation such as the shutter release, there exists a difference between the time at which the light measurement switch is closed and that at which the release switch is closed. By means of this difference, the electromagnetic release device is operated in a sequential way so as to obtain a proper photographing. However, for example, when the release button is pushed down abruptly, there is a possibility that before the output of the light measuring circuit has been stabilized after the light measurement switch is closed, the release switch is closed so as to start taking a picture.
That is, it is possible to constitute a light measuring circuit capable of making response to the abrupt alteration of the amount of the incident light by using a silicon photo-cell with quick photo-response characteristics as the light sensing element, whereby it takes a little time for the light measuring circuit to be stabilized, due to the stray capacity of the light sensing element, the amplifiers, the logarithmically compressing element and so on constituting the light measuring circuit. Further, a light sensing element with a quick photo-response characteristic is used, so that it is necessary to provide a low frequency band filter in order to eliminate the alteration in the light measurement output when the light source for illuminating the object contains a flicker, whereby the output of the light measuring circuit assumes the stabilized state after a certain time after the power source switch is closed because of the time delay in the low frequency band filter. Hereby, the time delay due to the afore-mentioned stray capacity is determined by the time during which the charge stored in the stray capacity is compensated by the photo-current of the light sensing element, whereby in the case of the small amount of the incident light, the produced photo-current is also small so that it takes a long time to reach the stabilized state while in the case of the large amount of the incident light it takes only a short time to reach the stabilized state. In the case of the afore mentioned time delay, the release button can be operated at a normal speed, while an abrupt operation of the shutter release button often leads to a misoperation.
Further, when the camera is used with a motor drive device or a remote control device it is always necessary to carry out the light measurement and the shutter release with a certain time difference in order to avoid improper exposure due to the afore-mentioned response delay of the light measuring circuit, for which the circuit construction becomes much complicated.
When a means for a certain amount of time delay is provided in order to avoid improper exposure due to the response delay of the light measuring circuit, particularly to cover a large time delay in the case of a dark object, the time difference for the shutter opportunity due to this operation delay becomes larger in the case of the bright object, whereby the number of pictures to be taken per second can not be increased during continuous photographing by means of a motor drive device.
Hereby, the time between the start of the current supply to the light measuring circuit and the stabilization of the circuit mainly depends upon the stray capacity of the light sensing element (SPC) and the logarithmically compressing element and the rising up time of the amplifier at the transient time and, further, the time delay due to the time constant of the low frequency band filter for eliminating the fluctuation of the light measurement output due to the flicker of the light source for illuminating the object when the photo-sensing characteristics of the light sensing element (SPC) is quick.